This invention relates to brazing filler alloys and more particularly to a relatively low temperature copper base brazing filler alloy useful for joining cemented carbide to steel.
Cemented carbides are used extensively in cutting tools, dies, mine tools and rock drills as a hardened cutting edge. In general, the cemented carbide part is relatively small and constitutes an insert in a larger steel holder. Although such inserts can be attached to the holder by several different techniques, brazing is preferred in mine tools and some rock drills.
The tool holder or shank of a mine tool is generally made of AISI 4340 or AISI 4130 steel. Desirable characteristics of a brazing filler alloy used to join a cobalt bonded cemented carbide insert to such a steel shank are good wetting characteristics for both the carbide and the steel without excessive penetration into the base metal, a resulting brazed joint with high shear strength and reasonable ductility to prevent separation of the insert from the holder during mining or drilling operations, and a sufficiently low brazing temperature, e.g., in the range of 815.degree.-900.degree. C. (1500.degree.-1650.degree. F.), that the desirable physical characteristics of the shank are not affected by the brazing operation.
Generally silver-base brazing filler alloys containing nickel, e.g., BAg-3 (in percent by weight, 50Ag, 15.5Cu, 15.5Zn, 3Ni, 16Cd) and BAg-4 (in percent by weight, 40Ag, 30Cu, 28Zn, 2Ni) or modifications thereof have been used in the past for brazing cemented carbides to steel. While these alloys have brazing temperatures within the desired range mentioned above, they do not wet cemented carbide well, necessitating additional processing including various special cleaning and surface treatment techniques. Some of the silver-base brazing filler alloys contain substantial amounts of cadmium, which is toxic and is a health hazard. Moreover, these alloys contain up to 50% by weight of silver which is affected by the high price of precious metals and by price fluctuations. In addition to the above, the shear strength of joints brazed with silver-braze filler alloys is fairly low, i.e., approximately 20,000 psi.
Among the non-precious brazing filler alloys which have been used for brazing cemented carbide to steel are BCu (pure copper), RBCuZn-D (in percent by weight 48Cu, 10Ni, 0.25P, balance Zn) and AMS 4764 (in percent by weight 38Mn, 9Ni, balance Cu). AMS 4764 has been observed to wet both the cemented carbide and the steel quite well. In addition, joints brazed with this alloy have very high shear strength and good ductility. The brazing temperature range, however, for all of the foregoing alloys is high, that is, 1093.degree.-1149.degree. C. (2000.degree.-2100.degree. F.) for BCu, 954.degree.-1010.degree. C. (1750.degree.-1850.degree. F.) for RBCuZn-D, and 950.degree.-1000.degree. C. (1742.degree.-1832.degree. F.) for AMS 4764. AISI 4340 steel used as the shank material, is normally austenitized in the range of 1500.degree.-1550.degree. F., oil quenched and tempered at 450.degree.-500.degree. F. for optimum strength and toughness. A high austenitizing temperature of 1750.degree. F. and above, which the steel undergoes if brazed with the aforementioned non-precious brazing filler alloys, leads to many problems. A high austenitizing temperature causes rapid grain-growth, which slows down the rate of transformation. As a result, more retained austenite is present in the steel, leading to increased warpage and cracking on quenching. A post brazed heat treatment is also essential when brazed at such high temperatures, necessitating an additional step in the process and adding to the cost of production.
British Pat. No. 996,177 published June 23, 1965 describes a nickel-copper-manganese alloy containing small amounts of boron and germanium as temperature depressants in addition to iron and silicon. All but two of the alloys described in this patent have melting temperatures higher than 1700.degree. F. and thus have a brazing range of 1750.degree.-1800.degree. F. Two of the compositions described as having melting temperatures of 1600.degree. F. contained boron which has been found to be unsuitable for cemented carbide brazing because it interacts with tungsten carbide to form brittle reaction products at the joint.
U.S. Pat. No. 4,071,538 issued to Ichiro Kawakatsu describes a copper base brazing alloy containing manganese, nickel, tin and indium. The patent states that the alloy is useful in high-temperature oxidation-resistant applications, however, and limits the manganese content to a maximum of 20% by weight. The brazing temperature for this alloy is 1000.degree.-1050.degree. C. (1836.degree.-1925.degree. F.) which is substantially higher than the maximum brazing temperature which can be tolerated in joining cemented carbide to steel.
This invention is directed to a non-precious metal brazing filler alloy for brazing cemented carbide to steel which overcomes the above difficulties.